In recent years, businesses such as Online to Offline (O2O) have been surging in which, through video or audio on a display screen of a television set or digital signage terminal, information related to the video or audio contents is distributed.
For example, the distribution of information through video enables users to obtain the content-related information by imaging video with a smart device such as a smartphone or tablet. The distribution of information through audio enables users to obtain the content-related information by recording audio with a smart device. Such business leads users to an online service or real shop through video or audio.
Technologies such as a video or audio recognition technology and a digital watermark technology which embeds other information in video or audio in a form which is hard to be perceived by humans are often used in order to realize such businesses. According to the video recognition technology, recognition with a higher accuracy can be obtained in less time because more feature amounts can be used in the case of video than audio which is a one-dimensional signal. According to the digital video watermark technology, more information can be embedded in less time in the case of video than audio.
From this point of view, the technology using video is more advantageous than that using audio. On the other hand, differently from audio, video causes a spatial distortion depending on a positional relationship between an imaging device and a display screen as a subject, and the distortion results in reduced accuracy of video recognition and of digital watermark detection. Thus, it is preferable that the spatial distortion be corrected by preprocessing.
With respect to preprocessing, a method for detecting points of four corners to correct a rectangular image of printed materials captured with a camera is known (see Patent Document 1, for example). According to this method, an edge is detected when a weighted sum of brightness adjacent to a point of interest of the rectangular image exceeds an edge determining threshold. A frame is detected when a weighted sum obtained from a proximity point range of the rectangular image, an optimal range width of proximity points, and the number of columns of a weighting factor matrix exceed a frame determining threshold. The coordinates of the four corners are then detected on the basis of the detected edge or frame.
With respect to preprocessing, a method for determining an image area to extract an original image area from an acquired image is also known (see Patent Document 2, for example). According to this method for determining an image area, end points of an edge line in the original image area included in the acquired image are obtained as a tentative vertex, the Fourier transform process is performed on the images around the tentative vertexes, and a phase angle of a low-frequency component is calculated. The misalignment of the tentative vertexes is adjusted on the basis of the calculated phase angle to obtain final detected vertexes, and the original image area is extracted on the basis of the coordinate values of the final detected vertexes.
A digital watermark embedding device which embeds watermark information in moving image data without deteriorating an image quality of moving image data is also known (see Patent Document 3, for example). This digital watermark embedding device periodically varies an area of watermark patterns superimposed on each image in the moving image data in chronological order and according to the values of symbols included in digital watermark information. According to the predetermined values of the pixels included in the watermark patterns, the digital watermark embedding device modifies the value of each pixel included in the area in which each of the images and the watermark patterns corresponding to the images are overlapped.
A digital watermark embedding method which permits a digital watermark detection from an acquired moving image in real time is also known (see Patent Document 4, for example). According to this digital watermark embedding method, a watermark pattern is generated using watermark information, frame display time, and watermark pattern switching information, and the watermark pattern is superimposed on a frame image of moving image data. A watermark embedded frame image group obtained by sequentially repeating such processes is combined to generate watermark embedded moving image data.
A watermark embedding method robust to de-flickering operations is also known (see Patent Document 5, for example). According to this watermark embedding method, each image is divided into at least a first and second image areas, and one value of a watermark sample is embedded in the image by increasing the global characteristic of the first image area and decreasing the global characteristic of the second image area. The other value of the watermark sample is embedded in the image by decreasing the global characteristic of the first image area and increasing the global characteristic of the second image area inversely.
A digital watermark embedding device for suppressing the flicker which occurs by embedding digital watermark information in a moving image is also known (see Patent Document 6, for example). This digital watermark embedding device generates a digital watermark pattern which includes a plurality of images having a first value, whose area varies in a constant cycle and in which a phase angle in a cycle variation of the area changes according to the values of symbols included in the digital watermark information. The digital watermark embedding device also generates a flicker suppressing pattern which includes a plurality of images having a second value, and whose area varies according to a frequency characteristic different from the cycle mentioned above. The digital watermark embedding device superimposes the digital watermark patterns and the flicker suppressing patterns on the image areas of each of the periodical images to modify the value of each pixel within the image areas on which the patterns are superimposed, depending on the first and second values.    Patent Document 1: Japanese Laid-open Patent Publication No. 2005-277732    Patent Document 2: Japanese Laid-open Patent Publication No. 2005-182164    Patent Document 3: Japanese Laid-open Patent Publication No. 2012-142741    Patent Document 4: International Publication Pamphlet No. WO 2007/015452    Patent Document 5: International Publication Pamphlet No. WO 2004/066626    Patent Document 6: Japanese Laid-open Patent Publication No. 2013-030974